Add uv-managed Ruff lint and ty type checks

[74th]

Motivation

• Provide a simple, uv-managed workflow to run linting with ruff and type checking with ty for the Python server and example apps.
• Fix issues discovered while running the tools (unused import and deprecated UTC usage) so the checks pass in CI/local dev.

Description

• Add a dev dependency group with ruff and ty in pyproject.toml and set ruff target to py313, and simplify the fastapi dependency spec to avoid invalid extras warnings.
• Document how to run lint/type-check with uv by adding a Lint / Type Check snippet to README.md.
• Fix example_apps/gemini.py by removing an unused asyncio import and reorder imports to satisfy ruff/ty.
• Replace datetime.utcnow() with timezone-aware datetime.now(UTC) in stackchan_server/ws_proxy.py and update the lockfile uv.lock to reflect the new tooling and dependency changes.

Testing

• Ran uv run ruff check stackchan_server example_apps --fix which completed with All checks passed! (success).
• Ran uv run ruff check stackchan_server example_apps which also passed (success).
• Resolved an initial ty configuration parsing error and then ran uv run --group example-gemini ty check stackchan_server example_apps which completed with All checks passed! (success).

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Codex Task